Compression method and air separation

Abstract

A compression method, a multistage compression system incorporating such method and an air separation method and plant utilizing such compression method and system in which a gas is compressed in a series of compression stages to produce a compressed gas and each of the compression stages incorporate a variable speed compressor. In such compression method and system, the compressed air is produced at a pressure that remains stable during both normal operational conditions and during turn down conditions during which the flow rate of the gas is reduced. This reduction is accomplished by reducing the speed of the compressor in an initial compression stage such that the compressor operates along a peak efficiency operating line at which the pressure ratio is directly proportional to the flow rate and such that the lower turn down flow rate is obtained at a reduced pressure which is made up in successive compression stages.

1 . A method of compressing a gas comprising:
 compressing the gas in a series of compression stages from a lower pressure to a higher pressure;   during normal operating conditions, supplying the gas from the series of compression stages at the higher pressure and at a higher flow rate and during turndown conditions, supplying the gas from the series of compression stages at the higher pressure and at a lower flow rate;   the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors; and   during the turndown operating conditions, adjusting the speeds of the variable speed motors and therefore, the speeds of the compressors such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.   
     
     
         2 . The method of  claim 1 , wherein during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages. 
     
     
         3 . The method of  claim 1 , wherein:
 during normal operating conditions, the gas is supplied from a final compressor at the pressure and at the higher flow rate and with an auxiliary compressor by-passed; and   during turndown operating conditions, the auxiliary compressor is in flow communication with the final compressor and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.   
     
     
         4 . The method of  claim 1 , wherein the gas is cooled between the compression stages and after having been compressed in the series of compression stages. 
     
     
         5 . The method of  claim 1 , wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors. 
     
     
         6 . The method of  claim 4 , wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors. 
     
     
         7 . A method of separating air comprising:
 compressing the air in a series of compression stages, with interstage cooling between the compression stages and after cooling to cool the air after having been compressed in the series of compression states, the air compressed within the compression stages from a lower pressure to a higher pressure;   during normal operating conditions, supplying the air at the higher pressure to a main heat exchanger from the series of the compression stages at a higher flow rate of the air and during turndown conditions, supplying the air to a main heat exchanger at the higher pressure from the series of the compression stages and at a lower flow rate;   cooling the air, after having been compressed, within the main heat exchanger and introducing the air into a distillation column system to produce return and product streams;   warming the return and product streams within the main heat exchanger to cool the air;   the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors; and   during the turndown operating conditions, adjusting the speeds of the variable speed motors and therefore, the speeds of the compressors such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.   
     
     
         8 . The method of  claim 7 , wherein during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages. 
     
     
         9 . The method of  claim 7 , wherein:
 during normal operating conditions, the gas is supplied from a final compressor at the pressure and at the higher flow rate and with an auxiliary compressor by-passed; and   during turndown operating conditions, the auxiliary compressor is in flow communication with the final compressor and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.   
     
     
         10 . The method of  claim 7 , wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors. 
     
     
         11 . A multistage compression system for compressing a gas comprising:
 a series of compression stages to compress the gas from a lower pressure to a higher pressure in a final stage of the compression stages;
 the multistage compression system configured to operate in a normal operating condition and in a turndown operating condition such that the air is supplied at the higher pressure from the compression stages and at a higher flow rate during the normal operating condition and the air is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown condition; 
   the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors and variable speed controllers connected to the compressors and configured to independently adjust the speed of the compressors; and   a master controller connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and in successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.   
     
     
         12 . The multistage compression system of  claim 11 , wherein the compression stages are configured such that during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages. 
     
     
         13 . The multistage compression system of  claim 11 , wherein:
 the compression stages have a final compressor in a final compression stage of the compression stages and an auxiliary compressor in an auxiliary compression stage of the compression stages;   a flow control network having a by-pass line, a first valve positioned between the by-pass line and an aftercooler connected to the final compressor and a second valve positioned between the aftercooler and the auxiliary compressor;   each of the first valve and the second valves operable to be set in a closed position and an open position such that during normal operating conditions, the first valve is set in the open position and the second valve is set in the closed position and the gas is supplied from a final compressor at the pressure and at the higher flow rate through the by-pass line and with the auxiliary compressor by-passed; and   during turndown operating conditions, the first valve is set in the closed position and the second valve is set in the open position such that the auxiliary compressor is connected to the aftercooler and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.   
     
     
         14 . The multistage compression system of  claim 11 , wherein intercoolers are positioned between the compression stages and an aftercooler is connected to the series of compression stages such that the gas is cooled after having been compressed in the series of compression stages. 
     
     
         15 . The multistage compression system of  claim 11 , wherein the variable speed motors are direct drive motors. 
     
     
         16 . The multistage compression system of  claim 14 , wherein the variable speed motors are direct drive motors. 
     
     
         17 . An air separation plant comprising:
 a series of compression stages to compress the air from a lower pressure to a higher pressure in a final stage of the compression stages;   intercoolers between the series of the compression stages and an aftercooler connected to the final stage of the compression stages;   the multistage compression system configured to operate in a normal operating condition and in a turndown operating condition such that the gas is supplied at the higher pressure from the compression stages and at a higher flow rate of the gas and the gas is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown conditions;   a main heat exchanger connected to the multistage compression system and configured to cool the air, after having been compressed;   a distillation column system configured to produce return and product streams, the distillation column system connected to the main heat exchanger such that the air, after having been cooled in the main heat exchanger, is introduced into the distillation column system and the return and product streams warm within the main heat exchanger to cool the air; and   a master controller connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and in successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.   
     
     
         18 . The air separation plant of  claim 17 , wherein the compression stages are configured such that during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages. 
     
     
         19 . The air separation plant of  claim 17 , wherein:
 the compression stages have a final compressor in a final compression stage of the compression stages and an auxiliary compressor in an auxiliary compression stage of the compression stages;   a flow control network having a by-pass line, a first valve positioned between the by-pass line and an aftercooler connected to the final compressor and a second valve positioned between the aftercooler and the auxiliary compressor;   each of the first valve and the second valves operable to be set in a closed position and an open position such that during normal operating conditions, the first valve is set in the open position and the second valve is set in the closed position and the gas is supplied from a final compressor at the pressure and at the higher flow rate through the by-pass line and with the auxiliary compressor by-passed; and   during turndown operating conditions, the first valve is set in the closed position and the second valve is set in the open position such that the auxiliary compressor is connected to the aftercooler and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.   
     
     
         20 . The air separation plant of  claim 17 , wherein the variable speed motors are direct drive motors.